Pain After I Run

I have focused my work on each of the events one by one.  First it was the bike because it was low impact and assisted me in losing weight.  Next, it was swimming for many of the same reasons.  I have gotten decent at both swimming and cycling and have now sifted my focus to running.  I used to run 30 minutes twice a week just so my body would have a little exposure to running and for the knowledge that I could run a 5K during sprints triathlons.  I am working on building a good base for running and I am running 20 minutes 4 times a week.  I have been doing this constantly for about a month and will start adding 10 percent a week to my runs.

The problem is that I continue to have pain on my left Gluteus Medius muscle after I run.  This is the one of the opposing hip flexor muscles.  My right side does not hurt only my left.  It feels like muscle soreness but I would have expected it to subside by now.  I have been told that it could be that my hip flexors have gotten stronger from biking and the Gluteus Medius muscle is over compensating and that the only reasons my left side hurts is because I am right dominate and they are stronger. I am looking for suggestions as to what I can do because it is hard enough to continue running without this nagging soreness.